AcousticsAcoustics

Reflection, Diffraction, Reflection, Diffraction, Refraction, DiffusionRefraction, Diffusion

Reflection of SoundReflection of Sound

If a sound is If a sound is activated in a room, activated in a room, sound travels sound travels radially in all radially in all directions. As the directions. As the sound waves sound waves encounter obstacles encounter obstacles or surfaces, such as or surfaces, such as walls, their direction walls, their direction of travel is changed, of travel is changed, i.e., they are i.e., they are reflected.reflected.

The diagram shows The diagram shows the reflection of the reflection of waves from a sound waves from a sound source from a rigid, source from a rigid, plane wall surface.plane wall surface.

Reflections from Flat SurfacesReflections from Flat Surfaces

Like a mirror, the reflected wavefronts act as Like a mirror, the reflected wavefronts act as though they originated from a sound image.though they originated from a sound image.

The image source is located the same The image source is located the same distance behind the wall as the real source is distance behind the wall as the real source is in front of the wall.in front of the wall.

Below 300 – 400 Hz, sound is best considered Below 300 – 400 Hz, sound is best considered as waves. Sound above 300 – 400 Hz is best as waves. Sound above 300 – 400 Hz is best considered as traveling in rays.considered as traveling in rays.

The mid/high audible frequencies have been The mid/high audible frequencies have been called the specular frequencies because called the specular frequencies because sound in this range acts like light rays on a sound in this range acts like light rays on a mirror.mirror.

Angle of ReflectionAngle of Reflection

Sound follows the Sound follows the same rule as light: same rule as light: the angle of the angle of incidence is equal incidence is equal to the angle of to the angle of reflection.reflection.

The pressure at the The pressure at the face of a perfectly face of a perfectly reflecting surface is reflecting surface is twice that of a twice that of a perfectly absorbing perfectly absorbing surface.surface.

Reflections from Convex Reflections from Convex SurfacesSurfaces

Reflection of plane Reflection of plane wavefronts of wavefronts of sound from a solid sound from a solid convex surface convex surface tends to scatter the tends to scatter the sound energy in sound energy in many directions.many directions.

This amounts to a This amounts to a diffusiondiffusion of the of the impinging sound.impinging sound.

Reflections from Concave Reflections from Concave SurfacesSurfaces

Plane wavefronts Plane wavefronts of sound striking of sound striking a concave a concave surface tend to surface tend to be focused to a be focused to a point.point.

The precision The precision with which sound with which sound is focused is is focused is determined by determined by the shape of the the shape of the concave surface.concave surface.

Reflections from Parabolic Reflections from Parabolic SurfacesSurfaces

A A parabolaparabola has the has the characteristic of characteristic of focusing sound focusing sound precisely to a point.precisely to a point.

This concept is This concept is used in microphone used in microphone design to create design to create highly directional highly directional microphones known microphones known as as parabolic micsparabolic mics..

Standing WavesStanding Waves

The concept of standing waves is directly The concept of standing waves is directly dependent on the reflection of sound.dependent on the reflection of sound.

A standing wave is a resonance condition A standing wave is a resonance condition in an enclosed space in which sound in an enclosed space in which sound waves traveling in one direction interact waves traveling in one direction interact with those traveling in the opposite with those traveling in the opposite direction, resulting in a stable condition.direction, resulting in a stable condition.

Reflection of Sound from Reflection of Sound from Impedance IrregularitiesImpedance Irregularities

Mismatches in impedance give rise to Mismatches in impedance give rise to reflections, which cause numerous undesirable reflections, which cause numerous undesirable effects, but can sometimes be desirable effects, but can sometimes be desirable effects instead.effects instead.

Example: when a sound wave (noise) traveling Example: when a sound wave (noise) traveling in an air conditioning duct suddenly in an air conditioning duct suddenly encounters the large open space of the room, encounters the large open space of the room, the impedance mismatch reflects a significant the impedance mismatch reflects a significant portion of the sound back toward the source. portion of the sound back toward the source. This is considered a desirable effect due to the This is considered a desirable effect due to the fact that the air conditioning noise is reduced.fact that the air conditioning noise is reduced.

Diffraction of SoundDiffraction of Sound

Wavefronts and rays of sound travel in Wavefronts and rays of sound travel in straight lines, except when something straight lines, except when something gets in the way.gets in the way.

Obstacles can cause sound to be Obstacles can cause sound to be changed in its direction from its original changed in its direction from its original path.path.

The process by which this change of The process by which this change of direction takes place is called direction takes place is called diffractiondiffraction..

Diffraction and WavelengthDiffraction and Wavelength

Obstacles capable of diffracting Obstacles capable of diffracting (bending) sound must be large (bending) sound must be large compared to the wavelength of the compared to the wavelength of the sound involved.sound involved.

The effectiveness of an obstacle in The effectiveness of an obstacle in diffracting sound is determined by the diffracting sound is determined by the acoustical sizeacoustical size of the obstacle. of the obstacle.

Acoustical size is measured in terms Acoustical size is measured in terms of the wavelength of the sound.of the wavelength of the sound.

Some Examples of DiffractionSome Examples of Diffraction In figure A, the upper edge of In figure A, the upper edge of

the wall acts as a new, virtual the wall acts as a new, virtual source sending sound energy source sending sound energy into the “shadow” zone into the “shadow” zone behind the wall.behind the wall.

In figure B, most of the sound In figure B, most of the sound energy is reflected from the energy is reflected from the wall surface, but that small wall surface, but that small portion going through the hole portion going through the hole acts as a virtual point source, acts as a virtual point source, radiating a hemisphere of radiating a hemisphere of sound into the “shadow” zone sound into the “shadow” zone behind the wall by diffraction.behind the wall by diffraction.

Diffraction by Large and Small Diffraction by Large and Small AperturesApertures

In figure A, In figure A, the arrows the arrows indicate that indicate that some of the some of the energy in energy in the main the main beam is beam is diverted into diverted into the shadow the shadow zone.zone.

Diffraction of Sound by Diffraction of Sound by ObstaclesObstacles

In figure A In figure A the obstacle the obstacle is so small is so small compared to compared to the the wavelength wavelength that it has that it has no no appreciable appreciable effect on the effect on the passage of passage of sound.sound.

Diffraction by Acoustic LensDiffraction by Acoustic Lens

Diffraction around the Human Diffraction around the Human HeadHead

For frequencies For frequencies 1 – 6 kHz 1 – 6 kHz arriving from arriving from the front, sound the front, sound pressure pressure changes around changes around the head.the head.

At frequencies At frequencies below 1 kHz, below 1 kHz, sound pressure sound pressure change is change is negligible.negligible.

Refraction of SoundRefraction of Sound

DiffractionDiffraction is changing the direction is changing the direction of travel of sound by encountering of travel of sound by encountering sharp edges and physical sharp edges and physical obstructions. obstructions.

RefractionRefraction changes the direction of changes the direction of travel of the sound by differences in travel of the sound by differences in the velocity of propagation.the velocity of propagation.

Refraction of LightRefraction of Light

The visual The visual distortion that distortion that occurs when an occurs when an object is placed object is placed in the water at in the water at an angle is due an angle is due to refraction of to refraction of the light waves the light waves in the denser in the denser medium.medium.

Refraction of Sound in SolidsRefraction of Sound in Solids

Sound waves Sound waves traveling traveling through through materials at an materials at an angle will bend angle will bend when they when they encounter a encounter a different different material of material of greater or greater or lesser density.lesser density.

Speed of SoundSpeed of Sound

Some Some examples examples of the of the speed of speed of sound sound based on based on density.density.

Refraction of Sound in the Refraction of Sound in the AtmosphereAtmosphere

The The atmosphere is atmosphere is anything but a anything but a stable, uniform stable, uniform medium for medium for the the propagation of propagation of sound.sound.

The diagram The diagram shows shows refraction due refraction due to temperature to temperature gradients.gradients.

Another ExampleAnother Example

This diagram This diagram shows shows refraction of refraction of sound radiated sound radiated upwards.upwards.

Note the Note the “sound “sound shadow” due to shadow” due to refraction near refraction near the ground.the ground.

Wind GradientsWind Gradients

OutdoorOutdoors, wind s, wind usually usually plays a plays a bigger bigger role in role in refractiorefraction of n of sound.sound.

Diffusion of SoundDiffusion of Sound

Sound diffusion is the spreading out or Sound diffusion is the spreading out or scattering of sound waves or rays in scattering of sound waves or rays in many different directions.many different directions.

Diffusion problems are most Diffusion problems are most troublesome in smaller rooms and at troublesome in smaller rooms and at the lower audio frequencies.the lower audio frequencies.

The problem with small spaces is that The problem with small spaces is that modal spacings below 300 Hz modal spacings below 300 Hz guarantee a sound field far from guarantee a sound field far from diffuse.diffuse.

The Perfectly Diffuse Sound The Perfectly Diffuse Sound FieldField

Even though unattainable, it is instructive Even though unattainable, it is instructive to consider the characteristics of a diffuse to consider the characteristics of a diffuse sound field. Randall and Ward have given sound field. Randall and Ward have given us a list of these characteristics:us a list of these characteristics:

– The frequency and spatial irregularities The frequency and spatial irregularities obtained from steady state obtained from steady state measurements must be negligiblemeasurements must be negligible

– Beats in the decay characteristic must Beats in the decay characteristic must be negligiblebe negligible

– Decays must be perfectly exponential Decays must be perfectly exponential (straight lines on a logarithmic scale)(straight lines on a logarithmic scale)

The Perfectly Diffuse Sound The Perfectly Diffuse Sound FieldField

– Reverberation time will be the Reverberation time will be the same at all positions in the roomsame at all positions in the room

– The characteristic of the decay will The characteristic of the decay will be essentially the same for be essentially the same for different frequenciesdifferent frequencies

– The characteristic of the decay will The characteristic of the decay will be independent of the directional be independent of the directional characteristics of the measuring characteristics of the measuring microphonemicrophone

Evaluating Diffusion in a RoomEvaluating Diffusion in a Room

The The loudspeaker loudspeaker and and microphone microphone were placed were placed in opposing in opposing corners corners because all because all room modes room modes terminate in terminate in the corners.the corners.

The The fluctuations in fluctuations in this response this response cover a range cover a range of about 35 of about 35 dB.dB.

Decay BeatsDecay Beats

As discussed As discussed earlier, decay earlier, decay beats beats induced by induced by closely closely spaced spaced modes cause modes cause irregularities irregularities in decay, in decay, especially at especially at low low frequencies.frequencies.

Exponential DecayExponential Decay

A truly A truly exponential exponential decay is a decay is a straight line on straight line on a level vs. time a level vs. time plot.plot.

This decay This decay shape is shape is probably a probably a mode or group mode or group of modes of modes encountering encountering low absorption.low absorption.

Another ExampleAnother Example

This decay This decay shape is due shape is due to an to an acoustically acoustically coupled coupled space.space.

Adjustable AcousticsAdjustable Acoustics

The The inconsistency of inconsistency of decay time at decay time at lower lower frequencies frequencies indicates a non-indicates a non-diffuse condition.diffuse condition.

Room ShapeRoom Shape The popularity of rectangular rooms is The popularity of rectangular rooms is

due in part to economy of construction, due in part to economy of construction, but it has its acoustical advantages.but it has its acoustical advantages.

The relative proportioning of length, The relative proportioning of length, width, and height of a sound sensitive width, and height of a sound sensitive room is most important.room is most important.

Some shapes, like parabola, can be Some shapes, like parabola, can be eliminated because they focus sound, eliminated because they focus sound, which is the opposite of diffusion.which is the opposite of diffusion.

Optimal Room ProportionsOptimal Room Proportions

This This table table shows shows several several “optimal“optimal” room ” room ratios as ratios as proposeproposed by four d by four different different studies.studies.

Optimal Room ProportionsOptimal Room Proportions

Another study Another study shows ideal room shows ideal room ratios. This study ratios. This study was done by Bolt was done by Bolt and is referred to and is referred to as the “Bolt Area.”as the “Bolt Area.”

Nonrectangular RoomsNonrectangular Rooms

Splaying one or two walls of a sound-sensitive Splaying one or two walls of a sound-sensitive room does not eliminate modal problems, room does not eliminate modal problems, although it might shift them slightly and although it might shift them slightly and produce somewhat better diffusion.produce somewhat better diffusion.

Flutter echoes definitely can be controlled by Flutter echoes definitely can be controlled by canting one of two opposing walls. The canting one of two opposing walls. The amount of splaying is usually between 1 foot amount of splaying is usually between 1 foot in 20 feet and 1 foot in 10 feet.in 20 feet and 1 foot in 10 feet.

Making the sound field asymmetrical by Making the sound field asymmetrical by splaying walls only introduces unpredictability splaying walls only introduces unpredictability in listening room and studio situations.in listening room and studio situations.

Geometrical IrregularitiesGeometrical Irregularities

Many studies have been made on what Many studies have been made on what type of wall diffusors provide the best type of wall diffusors provide the best diffusing effect.diffusing effect.

Geometrical diffusors must be at least 1/7Geometrical diffusors must be at least 1/7thth of a wavelength before their effect is of a wavelength before their effect is noticed.noticed.

Studies have shown that the straight sides Studies have shown that the straight sides of the rectangular shaped diffusor provide of the rectangular shaped diffusor provide the greatest effect for both steady-state the greatest effect for both steady-state and transient phenomena.and transient phenomena.

Rectangular Rectangular Wall PanelsWall Panels

Rectangular sound Rectangular sound absorbing panels absorbing panels with wood edges with wood edges are an inexpensive are an inexpensive way to achieve way to achieve both absorption both absorption and diffusion.and diffusion.

Convex vs. Concave SurfacesConvex vs. Concave Surfaces

The Poly vs. Planar DiffusionThe Poly vs. Planar Diffusion

Favorable Favorable reflection, reflection, absorption, and absorption, and reradiation reradiation characteristics characteristics favor the use of favor the use of the cylindrical the cylindrical surface over the surface over the planar surface.planar surface.

The Schroeder DiffusorThe Schroeder Diffusor

Manfred Manfred Schroeder Schroeder pioneered pioneered the concept the concept of acoustic of acoustic diffusors.diffusors.

The The diagram diagram shows his shows his first design.first design.

Commercial DiffusorsCommercial Diffusors

There are many companies now There are many companies now manufacturing various types of manufacturing various types of diffusors, available through pro audio diffusors, available through pro audio stores, mail order, or on-line sales.stores, mail order, or on-line sales.

The best diffusors are made by The best diffusors are made by companies with years of research and companies with years of research and experience backing their product.experience backing their product.

Choosing a product will depend on Choosing a product will depend on budget and specific diffusion needs.budget and specific diffusion needs.

Types of DiffusorsTypes of Diffusors

The most common types of The most common types of commercially available diffusors are:commercially available diffusors are:

– Reflection Phase-Grating (RPG)Reflection Phase-Grating (RPG)

– Quadratic-Residue (QRD)Quadratic-Residue (QRD)

– Diffractal (DFR)Diffractal (DFR)

Quadratic-Quadratic-Residue Residue DiffusorsDiffusors

RPG Diffusor RPG Diffusor Systems, Inc. is Systems, Inc. is a leading a leading manufacturer of manufacturer of diffusion diffusion devices.devices.

Dispersion PatternDispersion Pattern

The The hemidisc hemidisc pattern is pattern is shown here.shown here.

More RPG More RPG ProductsProducts

Three different Three different types of diffusors types of diffusors are shown here.are shown here.

Diffractal Diffractal DiffusionDiffusion

This design utilizes This design utilizes three different three different frequency ranges frequency ranges of diffusion.of diffusion.

Diffractal Diffractal DiffusionDiffusion

Another Another exampleexample

Diffusion in Three DimensionsDiffusion in Three Dimensions

Hemicylindrical and Hemispherical diffusion.Hemicylindrical and Hemispherical diffusion.